This invention relates to an image forming material of the type capable of forming visible images by physical development and to a method for image formation by the use thereof.
Physical development refers to a process wherein an image forming layer containing minute metal nuclei forming a latent image is treated with a developer containing metal ions to be reduced and a reducing agent (hereinafter frequently called "physical developer") to give a visible image constituted of metal particles and is generally known as a method for forming silver images.
Recently, in view of limited availability of silver salts from natural resources, recourse has been made to non-silver image forming materials. However, the only practical image forming method utilizing physical development is formation of a conductive pattern on a print substrate.
As an image forming method utilizing physical development proposed in the prior art, there is a method in which the reaction involving conversion of ferric ions to ferrous ions by photoirradiation is utilized, and noble metal ions are reduced by the ferrous ions formed to give metal developing nuclei, which are in turn subjected to physical development. The developing nucleus forming systems to be used in this method include iron-gold systems, iron-mercury systems and iron-silver systems (namely systems giving a brown print).
It has also been proposed to utilize an organic compound as a photoreducing agent. For example, according to one method, there is employed a photosensitizer system comprising a combination of diazosulfonate and a watersoluble mercury compound capable of forming nuclei. When this system is exposed to light, sulfite ions of diazosulfonate are liberated, by which mono-valent mercury salts disproportionate to give zero-valent mercury, on which mercury nucleus silver is precipitated when contacted with a silver-containing physical developer to produce a visible image. In place of diazosulfonate, other photosensitive compounds capable of liberating CN.sup.-, CNS.sup.-, NO.sup.- or S.sub.2 O.sub.3.sup.2- are also stated to be usable to obtain images (Japanese Patent Publication No. 3319/1962).
These methods, however, involve problems in treatment of waste liquors or in operation because mercury is employed.
Another method employs a photosensitizer system comprising a combination of a special diazonium salt such as hydroxybenzene diazonium salt and silver nitrate capable of forming nuclei. When this system is exposed to light, the diazonium salt is decomposed to produce phenol, and silver nuclei are formed by the reductive power of the phenol. A visible image can be obtained by contacting with a silver-containing physical developer. This is one example in which silver is used.
On the other hand, a research group to which we belong has already proposed a method in which a physical developer and a photosensitive resin are combined. That is, according to this method, light exposure and developing treatment are applied on a coating layer of a photosensitive resin to form a relief image through selective dissolution of the resin, simultaneous incorporation of metal nuclei in the relief image, followed by physical development. For formation of metal nuclei in the relief image, for example, the relief image may be treated first with an acidic aqueous palladium chloride solution and then with an acidic aqueous stannous chloride solution to form metallic palladium nuclei (Japanese Patent Application No. 102218/1980, filed on July 25, 1980).
This method achieves patternization of metallic latent images by utilizing the relief-pattern-forming capability of the photosensitive resin per se, but it involves a drawback in that sufficient resolution cannot be obtained because patternization is obtained as relief images.
The use of titanium oxide (U.S. Pat. Nos. 2,738,272 and 2,929,709), anthraquinone (U.S. Pat. No. 2,504,593) and tin chloride [Plating 58, 786(1971)]is also known and practiced, but each of these methods is utilized only as a pattern forming method on a print substrate.
In the case where titanium oxide is used, the products will have intransparent non-image portions. Although sodium anthraquinone-2,6-disulfonate is practically used in preparation of a print substrate, it is limited to a combination of a divalent copper salt such as cupric formate and cupric gluconate with nickel chloride, cobalt chloride or iron sulfate. Alternatively, when tin chloride is used, there are problems such as stannous chloride being readily oxidized by oxygen in the air to have a short life and the impossibility of using a light source with high power because the photosensitive wavelength is around 250 nm.